Back in the day, the hot rod magazines used to claim that electric motor winding paint (which I believe was glyptal) would seal up and bind to any loose sand from the casting process, that was missed in the cleaning process. Back then it likely didn't flake because powerful chemicals were used. Today the EPA has regulated most everything out of existence, so the good stuff that was once used is not available anymore.
In truth, a good cleaning is all that is needed. If you cannot get the surfaces that you can reach clean, then there is no chance of cleaning the
oil galleries. Since engines are not dropping like flies with sand in the bearings, it's not a problem.